The group A streptococci produce a large number of extracellular products, some of which may play a role in the pathogenesis of streptococcal infections and their sequelae. This proposal is addressed to a study of two of these products, streptolysin O (SLO) and streptolysin S (SLS). These cytolytic toxins have been investigated for many years, but the nature of their effects on the membranes of susceptible cells and their role in the pathogenesis of streptococcal infections is not understood. The interaction of SLO with mammalian cells will be studied with cultured cells that have been treated with certain oxygenated derivatives of cholesterol. Such cells are resistant to the effects of SLO and the correlation between toxin sensitivity and membrane cholesterol concentration will be examined. Attempts to define the membrane target of SLS will be made by studying toxin action on liposomes whose composition will be altered to contain various membrane lipids. SLS activity will be studied also on mammalian cells after altering the phospholipid composition of the cells. The early events in the initiation of streptococcal infections will be investigated using in vitro cell cultures. The mechanisms by which these organisms subsequently damage epithelial cells will be studied, and the role of SLO, SLS, and other extracellular products in this precess will be examined using mutants of group A streptococci.